Cost minimization is important to the large volume manufacture and application of Brushless DC (BLDC) motors and Switched Reluctance Motors (SRMs) in variable speed drives. BLDC motors are conventionally excited with bipolar currents that require a six-switch inverter. The unipolar motor needs fewer electronic parts and uses a simpler circuit than the bipolar motor. For these reasons, unipolar-driven motors are widely used in low- cost instruments. The savings in converter cost opens up many applications for variable speed drives (VSD) such as HVAC, fans, pumps and appliances, which have been dominated by constant speed drives.
A simple unipolar drive includes a single switch in series with each winding and a Zener diode or dump resistor in the freewheeling path. This drive is inefficient because the stored energy in the phases is dissipated. Better performance may be obtained by using topologies that have previously been used for driving Switched Reluctance Motors (SRM). An example is the C-dump converter, which offers full regenerative control. However, it has the disadvantage of requiring a complicated control for the dump capacitor voltage, the failure of which could be catastrophic. A buck converter-based drive for the unipolar BLDC motor has also been proposed. Both these topologies require a higher voltage on the dump capacitors than what is applied to the motor phases during turn-on. While this is a requirement for the SRM motor in order to achieve a fast turn-off of the phase current to avoid negative torque spikes, it is not so for the BLDC motor. A three- switch converter for the unipolar BLDC motor for ac supply operation was investigated, but it requires a modification in the machine windings and a split- capacitor voltage balancing control scheme.
For applications requiring operation from the utility supply, it is important to design the equipment to satisfy harmonics standards such as the IEC 1000-3-2, which limit the magnitude of current harmonics that can be injected into the utility. These standards are typically not satisfied by the conventional method of AC/DC conversion using a bridge rectifier followed by a large DC bus capacitor. Passive Power Factor Correction (PFC) circuits based on the use of reactive elements are impractical in 50–60 Hz single-phase lines because of size, weight and cost. Active PFC methods are becoming increasingly popular because of the availability of low- cost switches. They include a DC—DC converter between the diode bridge and the bulk capacitor, which is controlled such that the input current is shaped to follow the input voltage. The frequency spectrum of the input current would then consist of the fundamental plus easily filtered higher order harmonics. For low power levels, the extra cost and complexity of the additional PFC stage is not justified by the improvement in power factor.